1. Field of the Invention
The present invention relates to a sheet type electrode for use in the measurement of ions, for example, in the measuring of an ion-concentration, such as pH.
2. Description of the Technical Background
Recently, an electrode for measuring an ion-concentration,, such as pH, has been formed in a sheet-like shape in order to miniaturize the construction of the electrode, reduce the cost of production and to improve its operation and maintenance.
FIG. 7(A) discloses an external appearance of a composite sheet type electrode for use in the measurement of pH as described in a Japanese patent application by the present applicant (Horiba, Ltd.) filed on Nov. 28, 1986, and FIG. 7(B) shows a section of the measuring electrode of FIG. 7(A). Referring now to both FIG. 7 (A) and FIG. 7(B), reference numeral 61 designates a substrate formed of a material having a sufficiently high electrical insulating property even when immersed in a solution containing electrolytes. Reference numeral 62 designates an electrode formed on an upper surface of the substrate 61, the electrode 62 being formed by subjecting it to an appointed pretreatment and then a silk screen printing with an Ag paste. A part of the electrode 62 is formed as an internal electrode coated with an electrode material, such as for example AgCl, and the remaining part of the electrode 62 is formed as a lead portion 64.
Reference numeral 65 designates a support layer provided with a through hole 66 at a place corresponding to the internal electrode 63 and formed of a material having a sufficiently high electrical insulating property even when immersed in a solution containing electrolytes, such as for example polyethylene terephthalate. The support layer 65 is formed on the substrate 61 while exposing the lead wire 64 at its circumference. and a gel-evaporation inhibitor, such as glycerine, to a basic internal solution prepared by adding a phosphoric acid-buffering agent to, for example, a 3.3 M-aqueous solution of KCl supersaturated with AgCl. The internal solution 67 is placed into the through hole 66 by heating it to turn it into a paste and then screen printing it so that its upper surface may project slightly over the upper surface of the support layer 65 in a free condition and also overlap on the internal electrode portion 63.
Reference numeral 68 designates a flat plate-like response membrane having its lower surface contacting an upper surface of the gelatinized internal solution 67. The gelatinized internal solution 67 is sealed in the through hole 66, and the membrane 68 is fixedly mounted on the support layer 65 along its circumference by the use of an adhesive material 69 (for example, organic high molecular adhesives of silicon-, epoxy-, urethane- and the like series containing silane coupling agents and the like).
Referring to FIG. 7(A), reference numeral 70 designates a liquid junction membrane formed of porous inorganic sintered materials or porous organic high molecular materials impregnated with KCl having its lower surface contacting an upper surface of the other gelatinized internal solution. The membrane 70 is adhered to an upper surface of the support layer 65 along its circumference. The internal construction of the liquid membrane portion 70 is nearly the same as that of the measuring electrode shown in FIG. 7(B). Reference numeral 71 designates a holder portion to retain a sample solution under test.
With the above described construction, for example in a measuring electrode assembly, the response membrane 68, the internal electrode portion 63 and the lead portions 64 are all formed on the same surface of the substrate 61 so that the following disadvantages may occur. For example, the sample solution can run over the holder portion 71 or the sample solution can be inadvertently dropped. As a result, the sample solution can adhere to the lead portions 64, whereby a poor insulation can occur in the lead portions 64. When the response membrane 68 is permeable to light, a further disadvantage occurs in that an outside light is directly incident upon the internal electrode portion 63, whereby the measurement is influenced by the light.
Also, a reference electrode assembly can have similar problems.